[C#] Rewrite trait impl based on typescript, not Java

[C#] Rewrite trait impl based on typescript, not Java

C# doesn't let us pass arbitrary C# objects over the FFI as a
pointer, at least not easily, so instead we need something that
looks much more like what we do for TypeScript.

Here we rewrite the trait handling in C# based on the TypeScript
code and instead store weak pointers so we can reference trait
implementation instances by a single `long`.